What are hygrometers and humidifiers for violin cases?

I am doing a orchestra project on violin cases and the additions to normal cases include hygrometers and humidifiers. Does anyone have any information or history on both of the instruments? How do they work?What are hygrometers and humidifiers for violin cases?
A humidifier is not a suitable device for a violin case. Please see the reference to ';Dampit'; below.





A hygrometer is a device to measure that relative humidity in the air. This is an important article to keep in a violin case, since the instrument will tend to crack when the humidity is low.





Inexpensive hygrometers can be bought at a cigar shop, or at a music store, or as a part of a home weather station. They can be made in various ways:





Mechanical - A hair changes length with humidity. In your Great Aunt's old weather station in the parlor, there is probably a single blonde horse hair. These are quite inaccurate.





Chemical - Cobalt chloride is used in cardboard humidity indicators like the strip included with the ';Dampit'; product. The color changes with humidity. These are even more inaccurate than the ';hair'; hygrometers. Here is a link to Dampit:


http://www.dampits.com/





Electronic - There are integrated circuits based on voltage changes within certain materials, such as these:


http://content.honeywell.com/sensing/pro鈥?/a>





It is my advise not to buy a hygrometer as part of a case.


When (not if ) the glue disintegrates holding it to the case lining, it will rattle around with the violin and possibly scratch it. A hygrometer for cigar humidores can be bought for under $20, and kept in the pocket of the case. Make sure you have a calibration screw on the device, and calibrate it with a known electronic model.What are hygrometers and humidifiers for violin cases?
Im not positive but i believe that a hygrometer measure humidity in the air and of course an humidifier would disburse humidity into the air. The need for these in a violin case would be to keep a certain amount of humidity in the case and also be able to monitor it to preserve the instrument and also to help keep it in tune musically.
See:





What is the best way to protect the violin from extremes of temperature and humidity?


http://beststudentviolins.com/StringCare鈥?/a>